1. Field of the Invention
This invention relates to acoustooptic light deflection systems, and more particularly to apparatus for deflecting two or more beams of light of different wavelength through substantially the same scan angle.
2. Description of the Prior Art
It is well known that the passage of acoustic waves through a transparent medium produces a diffraction grating by periodic variation of the index of refraction of the medium. Light directed through the medium, or cell, is diffracted through an angle such that the angle between the undiffracted and diffracted beams is .alpha., where: EQU .alpha..congruent..lambda.f/v (1)
for small values of .alpha. and where .lambda. is the wavelength of the illuminating light, f is the frequency of the acoustic wave, and v is the acoustic velocity.
It can be seen that for a given frequency f equation (1) defines a different angle .alpha. for each wavelength making up a multi-color light beam. Thus, for polychromatic beams, each different wavelength component will be diffracted at an angle which differs from that of any other component. If frequency f is continuously varied through a bandwidth .DELTA.f, each component of the light beam will scan through a unique scan angle .DELTA..alpha. given by EQU .DELTA..alpha.=.lambda..DELTA.f/v (2)
This situation has been summarized in FIG. 1, wherein a conventional diffraction cell 10 contains acoustic waves 12 of frequency f.sub.1 produced by a transducer 14, which is in turn energized by a signal source 16. A light beam 18 having two wavelength components .lambda..sub.1 and .lambda..sub.2 (.lambda..sub.2 &gt;.lambda..sub.1) enters the cell and is diffracted by the acoustic wavefronts. Since the angle of diffraction is a function of optical wavelength, the light of each wavelength exits at a different respective diffraction angle.
If the frequency of acoustic waves 12 was changed from f.sub.1 to a higher frequency f.sub.2, the diffraction angles would change in accordance with equation (2) by .DELTA..alpha..sub.1 and .DELTA..alpha..sub.2, respectively, .DELTA..alpha..sub.2 &gt;.DELTA..alpha..sub.1. Thus, it is apparent that, even if the diffraction paths of the two wavelengths could be made co-linear at one chosen frequency such as for example by impinging beams of different wavelengths at different respective incident angles, they would not remain co-linear as the frequency was varied.
Further, it may be appreciated from FIG. 1, that the smaller the difference in wavelength between .lambda..sub.1 and .lambda..sub.2, the smaller the difference between the diffraction angles .alpha..sub.1 and .alpha..sub.2 for a given frequency f. For conventional acoustooptic cells, the difference between the diffraction angles of a beam of red light (e.g. 6300 A) and a beam of blue light (e.g. 4400 A) is less than 0.5.degree.. Thus, when it is desired to sweep the beams through a scan angle greater than 0.5.degree., the scan angle for the blue beam will begin to overlap the scan angle of the red beam. For many applications, it is desirable to sweep the light beams emerging from the acoustic cell through a scan angle of at least 3.degree.. Thus, for the above example, the scan angle of the blue beam will overlap the scan angle of the red beam by 2.5.degree..
In co-assigned U.S. Pat. No. 3,783,185, which issued Jan. 1, 1974, to R. A. Spaulding, an optical system was disclosed for scanning a composite output beam made up of a light of a plurality of optical wavelengths horizontally across a constantly advancing web of photographic paper. One of the horizontal scanning systems of that disclosure includes a rotating mirror which reflects the composite beam and sweeps it across the web. In many printer applications, such beam scanning might be done with acoustooptic deflection cells, and in such event, some method for compensating for the dispersive characteristics of deflection is necessary when polychromatic light is to be deflected.
In their article "Equalization of Acoustooptic Deflection Cells in a Laser Color TV System", Applied Optics, Vol. 9, No. 5, May 1970, W. H. Watson and A. Korpel recognized that the dispersive characteristics of acoustooptic deflection mandates some form of electrical or optical compensation before three primary color images will remain in register as the acoustic frequency is varied over a bandwidth .DELTA.f. They effect such compensation optically by using separate compensating prisms to magnify the smaller blue and green deflection angles to match that of the red beam. This approach requires that the individual beams be spatially separated and, to produce such spatially separated beams, Watson and Korpel use three separate acoustooptic deflectors which are driven by the same signal.